It is normal in the control of multi-axis equipment to perform an interpolation process where the distance travelled by each axis over the next time period is calculated by a master processor (interpolator) and transmitted to the corresponding axis controller. These processors do not guarantee that the motion generator will follow the prescribed path and errors are generated. Obviously, it would be preferred to have each axis follow exactly the prescribed path or failing this, to have each axis lag behind the command by a constant time increment. The latter may be achieved by specifying an allowable error distance which is proportional to the commanded velocity. This has been achieved in prior systems by a number of different means. The most common are the following:
1. Complex digital algorithms which attempt to cancel the fundamental dynamics of each axis. These algorithms operate on the incoming commands from the interpolator and manipulate the input so that the final motion is close to that required. A survey of such methods is provided in Tomizuka, M., 1987, "Zero Phase Error Tracking Algorithms for Digital Control", ASME Transaction, Journal of Dynamic Systems, Measurement and Control, Vol. 109, March, pp 65-68. This approach has many advantages but is computationally intensive and has a major disadvantage in that changes in the characteristics of the drive will lead to large errors. Such changes in the dynamics are particularly evident in the presence of non-linearities or limits such as amplifier saturation.
2. Another approach which avoids the problem of exceeding the capacity of the system is to process the position file, generally using higher order splines, to ensure that the acceleration and jerk are within acceptable limits. This procedure is even more time consuming and relies on extremely good process and dynamic models, thus, cannot accommodate unexpected disturbance
inputs such as changes in cutting forces or changes in axis characteristics as a result of work piece mass, etc.
U.S. Pat. No.5,222,017 issued Jun. 22, 1993 to Yellowley et al. describes a particular control system utilizing a state line, the condition of which is used to trigger upgrading of the action of the control axis slaves to the next increment of the spline only when the state line indicates that all parameters influencing the state line have been satisfied. If all parameters are not satisfied, the status of the state line indicates that an advance is not acceptable to at least one of the monitored parameters and the last increment of movement is repeated by all of the axes until the condition triggering the status of the state line has been overcome, i.e. the state line indicates an acceptable status and only then do the slaves all advance to perform the next increment of their respective actions.
Generally, with the Yellowley et al. system, triggering of the state line from an accept status to a hold status is dependent on the amount of measured error compared with a predetermined or preset acceptable error limit which, when exceeded, triggers or flags the state line to a hold status to prevent upgrading of the slave axis to the next spline increment.
A modification of the system described in U.S. Pat. No. 5,222,017 is described in application 08/100,810 filed Aug. 2 1993 Yellowley et al. This system is a more sophisticated system, wherein the degree of error will trigger the status of different state lines to define the size of the increment of advance of the operation that may be used.